DESCRIPTION (Applicant's abstract): Long term complications of type 1 diabetes mellitus are reduced in patients with tightly controlled plasma glucose levels. A frequent consequence of this tight control is repeated episodes of hypoglycemia, ultimately leading to hypoglycemia associated autonomic failure and unawareness. Reduced conscious perception of hypoglycemia coupled with reduced secretion of counterregulatory hormones places patients at risk for deep and prolonged hypoglycemic episodes that can lead to seizures, coma, or death. This proposal describes a rodent model in which a single episode of hypoglycemia reduced sympathoadrenal responses to subsequent hypoglycemic episodes. Brain alterations observed after one bout of hypoglycemia include reduced expression of the neuropeptides NPY and POMC in the arcuate nucleus of the hypothalamus, and evidence of apoptosis in arcuate neurons. The present proposal has three aims directed to determining the mechanism and functional significance of arcuate damage and reduced counterregulation in rats. The first aim is to assess whether hypoglycemia-induced arcuate changes and suppressed counterregulatory responses are related specifically to hypoglycemia, or whether hyperinsulinemia can induce similar changes in the absence of hypoglycemia. A glucose clamp will be used to induce hyperinsulinemic euglycemia or hyperinsulinemic hypoglycemia to distinguish between these possibilities. The next aim is to use hypothalamic microinjections of 2-deoxy-D-glucose to study the arcuate and ventromedial nuclei of the mediobasal hypothalamus, comparing and contrasting the roles of these nuclei in behavioral and endocrine responses to glucoprivic stimulation. In the last aim, the potential functional significance of hypoglycemia-induced arcuate changes will be assessed. Behavioral and endocrine responses to glucoprivic stimulation of the arcuate nucleus after antecedent hyperinsulinemia/hypoglycemia will be measured as an indicator of the glucose sensitivity of the hypothalamus after hypoglycemia. The outcome of these studies will provide information about hypothalamic mechanisms of hypoglycemia detection and about hypothalamic alterations that may contribute to the phenomena of hypoglycemia unawareness and hypoglycemia-associated autonomic failure.